An eyepiece is a type of lens system that can be attached to a variety of optical devices such as telescopes and microscopes. An objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image, with the amount of magnification generally being dependent on the focal length of the eyepiece. An eyepiece typically includes several lens elements. The entrance pupil of the eyepiece is external or located outside of the eyepiece system (i.e., on the objective or object space side of all the lens elements making up the eyepiece). Generally, an eyepiece is designed for optimal performance for a specific distance to this entrance pupil (i.e. with minimum aberrations for this distance). The distance between the entrance pupil and the first lens of the eyepiece can vary greatly depending on the design and intended application of the eyepiece. For example, in a refracting astronomical telescope the entrance pupil may be several feet distant from the eyepiece; whereas in a microscope the entrance pupil may be only a few inches from the eyepiece. More generally, any optical system with a similar external entrance pupil can be termed eyepiece-like, and in common with classical eyepieces, such optical systems, because of the lack of symmetry about that external pupil, are known to be difficult to correct for off-axis aberrations, notably coma, astigmatism, and lateral color.